Solid lubricating, hard and fracture resistant composites for surface engineering applications

ABSTRACT

A method and composition for producing a monolithic hybrid composite consisting of a composite matrix, a reinforcement phase, and a solid/self-lubrication phase. The composition may comprise a graphite (“C”) phase and a titanium carbide (“TiC”) phase in a nickel (“Ni”) matrix. The process may include the step of blending pure Ni and pure titanium (“Ti”) powders with Ni-coated graphite powder using Laser Engineered Net Shaping (“LENS”). The novel composite, when achieved with an optimum chemical and structural phase ratio, exhibits a balance of high hardness, fracture toughness, and low friction/wear,

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims, and is entitled to the, priority to U.S.Provisional Patent Application Ser. No. 61/613,341, entitled SOLIDLUBRICATING, HARD AND FRACTURE RESISTANT COMPOSITES FOR SURFACEENGINEERING APPLICATIONS, filed on Mar. 20, 2012, the entire content ofwhich is hereby incorporated by reference.

STATEMENT OF GOVERNMENT RIGHTS

The present invention was partially funded by Federal Grant NSFCMMI-1100648. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the field of compositeshaving utility as coatings deposited on engineering materials. In someembodiments, the invention relates to monolithic hybrid compositesexhibiting solid/self-lubrication, high mechanical hardness, and highfracture toughness.

BACKGROUND

Coatings deposited on engineering materials frequently experiencemechanical wear, delamination, and/or fracture. Thus, compositessimultaneously exhibiting the properties of solid/self-lubrication, highmechanical hardness, and high fracture toughness, are needed for coatingengineering materials.

The present invention solves this problem by providing, in certainembodiments, monolithic composites exhibiting solid/self-lubrication,high mechanical hardness. and high fracture toughness. Metalmatrix/hybrid composites are highly versatile engineering materials inwhich a metal is combined with two or more non-metallic phases to yielda novel material that has superior engineering properties such as highhardness, high fracture toughness, and low wear rates. Such materialsfind utility in many applications ranging from aerospace, drilling, windenergy, and land-based turbines and compressors.

The process of the present invention may include the use of a LaserEngineered Net Shaping (“LENS”) process to make a hybrid monolithiccomposite that merges solid/self-lubrication, high hardness, and highfracture toughness properties. This process allows the composite to beprocessed near net shape, and is a flexible process that allows fortailoring of the material phases both chemically and structurally, whichcannot be accomplished using conventional laser cladding and hard facingtechniques. Near net shape indicates that the initial processing of thecomposite is very close to its final (net) shape to be used in service,without the use of additional surface finishing operations such asmachining or grinding.

SUMMARY

The present invention relates generally to hybrid monolithic compositesexhibiting characteristics of solid/self-lubrication, high hardness, andhigh fracture toughness, and a method for fabricating hybrid monolithiccomposites.

In certain embodiments, the invention comprises a novel, bulk hybridcomposite for surface engineering applications comprising a graphite(“C”) phase, a titanium carbide (“TiC”) phase, in a nickel (“Ni”)matrix.

In certain embodiments, the hybrid monolithic composite of the presentinvention is produced using a LENS process.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1 shows a ternary phase diagram of Ni, Ti and C. Compositions A andB are Ni-10Ti-10C and Ni-3Ti-20C, respectively, and are shown on thephase diagram.

FIG. 2 shows X-ray diffraction of the Ni-3Ti-20C composite. Thestructural phases were determined to be nickel (face-centered cubiccrystal structure), titanium carbide (rocksalt crystal structure), andcarbon (graphite allotrope).

FIG. 3 shows a planar surface Auger electron spectroscopy map of theunworn Ni-3Ti-20C composite. The chemical phases were determined to beNi, TiC and C.

FIG. 4 shows friction coefficient and wear rates/factors of theNi-10Ti-10C and Ni-3Ti-20C composites.

FIG. 5 shows planar surface Auger electron spectroscopy map of the wornNi-3Ti-20C composite. Tribochemical formation of an in situ, lowinterfacial shear strength carbon film is determined inside the weartrack.

FIG. 6 shows scanning electron microscope image of the subplanar(subsurface) focused ion beam cross-section of worn Ni-3Ti-20Ccomposite. Sheared graphite that comes to the surface is shown thatproves the solid/self-lubrication of the composite.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates generally to monolithic hybrid compositesexhibiting solid/self-lubrication, high mechanical hardness, and highfracture toughness. In certain embodiments, the invention comprises anovel, monolithic hybrid composite for surface engineering applicationscomprising: a graphite (“C”) phase, and a titanium carbide (“TiC”)phase, in a nickel (“Ni”) matrix. In some embodiments. LENS is used tomake the monolithic hybrid composite. The LENS process allows thecomposite to be processed at near net shape, while allowing fortailoring of the material phases both chemically and structurally.

Broadly, metal matrix/hybrid composites are highly versalite engineeringmaterials in which a metal is combined with one or more non-metallicphases to yield a novel material that has desired engineeringproperties.

Atomic ratio as used herein is defined as a ratio between the atomicweight percents of the given components.

Composition for Surface Engineering Applications

In one embodiment, the invention provides a monolithic hybrid compositeconsisting of a composite matrix, a reinforcement phase, and asolid/self-lubrication phase.

The composite matrix has properties of high fracture toughness (>50MPa√m), high stiffness (>150 GPa elastic modulus), high yield strength(>150 MPa), high tensile strength (>400 MPa). and high melting point(>1000° C.). Examples of the composite matrix include nickel, Ni, and/ortitanium, Ti. The composite matrix consists of a pure metal or alloy,which, in general, exhibit high fracture toughness in comparison tointermetallics, ceramics and polymers.

The chemical bonding in this phase should be metallic and notintermetallic (mixed bonding between metallic and ionic/covalent), sinceionic/covalent character can lead to brittleness. If additional metallicphases are added to the composite during the LENS process, they shouldhave low chemical reactivity with other phases during solidification,since they may form unwanted chemically-alloyed phases. Thus,intermetallic bonding and high reactivity may result in unwantedchemical phases with detrimental properties.

The reinforcement phase has properties of high melting point (>1000°C.), high stiffness (>300 GPa elastic modulus), high tensile strength(>500 MPa), and high hardness (>1000 VHN, Vickers hardness number).Examples of the reinforcement phase include titanium carbide, TiC,and/or titanium nitride, TiN, titanium boride (TiB), or any otherrefractory metal carbide, nitride, or boride. The ceramic reinforcementphase consists of a refractory metal carbide, nitride, or boride, which,in general, exhibit high hardness and high stiffness in comparison tometals, intermetallics, and polymers.

The chemical bonding in this phase, e.g. Titanium Carbide (“TiC”) shouldbe ionic/covalent mixture with mostly covalent bonding to promote highlattice energy. The chemical reactivity with other phases duringsolidification to form this compound, or others, should be high.

The self-lubrication phase has properties of high melting point (>1000°C.), low steady-state sliding friction coefficient (<0.1) and low wearrates/factors (<10⁻⁷ mm³/Nm) under a variety of loading conditions, lowstiffness (<10 GPa elastic modulus along at least one crystallographicdirection, which allows for easy mechanical shear), and low hardness(<20 VHN, Vickers hardness number). Examples of thesolid/self-lubrication phase include graphitic carbon, C, and/ormolybdenum disulphide, MoS₂. These phases are termed solid lubricants,which, in general, exhibit low friction, low wear, low elastic modulus,and low hardness.

The chemical bonding in this phase should be mixed between weakout-of-plane secondary bonding, like Van der Waals bonding thatundergoes easy shear, and strong in-plane primary bonding, like covalentbonding. The chemical reactivity with other phases during solidificationshould be relatively high, since chemical phase formation with the basemetal (Ti or Ni) will result in metal carbide formation, e.g. TiC. Inaddition, the self-lubricating phase, e.g. graphite (“C”), should notentirely transform into the TiC phase as to retain its self-lubricatingproperties.

The overall thickness of the composite should be about ≧5 mm. The sizeof the primary graphite (“C”) self-lubricating phase in the compositeshould be about ≧1 μm, and the size of the Titanium Carbide (“TiC”)reinforcement phase in the composite should be about ≧0.5 μm.

The composition of the present invention exhibits a balance of highhardness, fracture toughness, and low friction/wear, and is suitable formany moving mechanical assembly applications including oil-drillingcomponents such as wear bands, stabilizers, drill collars for tunnelboring, and land-based turbines and compressors.

In some embodiments, the raw materials include near spherical, pure Ni(40-150 μm diameter), pure Ti powder (40-150 μm diameter), and Ni-coatedgraphite powder (40-150 μm diameter) [Crucible Research]. The term nearspherical powder refers to gas inert atomized particle that isnear-spherical in shape. The purity of the Ni and Ti powders are˜98-99.5% pure. In some embodiments, the ratio of Ti to C to Ni is 3atomic % Ti, 20 atomic % C, and 77 atomic % Ni, referred to asNi-3Ti-20C. FIG. 1 shows a ternary phase diagram of the Ni, Ti and Cphases. Compositions A and B shown on the phase diagram are atomicratios Ni-10Ti-10C and Ni-3Ti-20C, respectively. The acceptable ranges(in atomic weight percent) for Ni, Ti and C are from ˜75 to 82, 3 to 5,and 15 to 20, respectively.

Method of Fabricating a Monolithic Composite

In certain embodiments, the present invention provides a method forfabricating a monolithic hybrid composite comprising the step of:blending elemental Ni powder and titanium (“Ti”) powder together withnickel-coated graphite powder. In some embodiments, the hybridmonolithic composite is fabricated using pure spherical Ni (40-150 μm),pure Ti (40-150 μm), and Ni-coated graphite powder (about 40-150 μm indiameter).

In one embodiment, the method of the present invention comprisesblending Ni and Ti powders with Ni-coated graphite powder. In thisembodiment, the components may be in a ratio of 3 atomic % Ti. 20 atomic% C, and 77 atomic % Ni (Ni-3Ti-20C), shown in FIG. 1. The acceptablerange, in atomic ratio, of self-lubrication phase (“C”) to reinforcementphase (“TiC”) is about 7 to 4.

In certain embodiments, monolithic hybrid composites are laser processedvia Laser Engineered Net Shaping (“LENS”). These monolithic hybridcomposites may be comprised of non-metallic titanium carbide (“TiC”) andgraphite (“C”) reinforcements in a metallic nickel (“Ni”) matrix.

In one embodiment, the monolithic hybrid composite of the invention isfabricated using a LENS process, wherein the monolithic hybrid compositeis fabricated based on direct laser deposition. Similar to rapidprototyping technologies such as stereolithography, the LENS processbegins with a computer-aided design (CAD) design file of athree-dimensional component, which is sliced into series of layerselectronically. The LENS system uses nozzles which direct a stream ofmetal powder at a moveable central point while a high powered laser beamheats that point. The substrate is continuously moved, guided by thedata from the CAD model, and layer-by-layer the nozzles and laser worktogether to build up the three-dimensional part. The detailed mechanismof this process has been reported in references [1-3].

In one embodiment, a nickel plate substrate was used for depositing theNi-Ti-C composites with atomic ratios shown in FIG. 1. A high powered500 W Nd:YAG laser, emitting near-infrared laser radiation at awavelength of 1.064 μm, is focused on the substrate to create a meltpool into which the powder feedstock is delivered through an inert gasflowing through a multi-nozzle assembly. The nozzle is designed suchthat the powder streams converge at the same point on the focused laserbeam. Subsequently the substrate is moved relative to the laser beam ona computer-controlled stage to deposit thin layers of controlled widthand thickness. The scan speed of the Nd:YAG laser was about 10inches/min and the hatch width used for the deposition was about 0.018inch with about 0.01 inch of layer thickness.

The LENS process produces near-net shaped components that do notnecessarily require secondary rough machining. Moreover, the LENSprocess is an industrial scale tool, which works automatically andwithout constant supervision. Furthermore, LENS parts exhibit close totheoretical bulk density, and have excellent as-fabricated mechanicaland tribological (friction and wear) properties.

In some embodiments, the LENS process can be used to apply a surfacelayer of the hybrid monolithic composite of the present invention to apre-existing component. In some embodiments, the LENS process can beused to fabricate a near net shape functionally andcompositionally-graded component wherein the surface composition has thecombined high hardness, fracture toughness, and excellentsolid/self-lubrication.

Example 1

Based on the x-ray diffraction structural analysis of the Ni-3Ti-20Catomic ratio composite shown in FIG. 2, the structural phases weredetermined to be nickel (face-centered cubic crystal structure),titanium carbide (rocksalt crystal structure), and carbon (graphiteallotrope). FIG. 3 is an Auger electron spectroscopy chemical map of theunworn Ni-3Ti-20C composite that shows a typical surface distribution ofthe Ni, TiC and C chemical phases. These chemical phases are inagreement and complement the x-ray diffraction determined structuralphases.

The effect of the C/Ti atomic ratio of the powder mixtures shown in FIG.1 was found to have a dominant influence on the microstructure,microhardness, and tribological properties of the composites. Evaluationof monolithic hybrid composites revealed that the volume fraction of theprimary titanium carbides (TiC), which form during solidification,versus primary graphite (“C”), changes substantially as a function ofthe C/Ti ratio. The presence of primary graphitic carbon enhanced thesolid/self-lubricating behavior of these composites resulting in anoptimum combination of hardness and lubricity while still maintainingthe high fracture toughness inherent to the Ni matrix, which isdesirable for many surface engineering applications.

To achieve this balance, a composite powder was produced with acomposition of 3 atomic % Ti, 20 atomic % C, and 77 atomic % Ni, theNi-3Ti-20C composite shown in FIG. 1. FIG. 4 shows the improvement infriction coefficient and wear rates/factors of the Ni-3Ti-20C compositein comparison to the Ni-10Ti-10C composite and baseline LENS depositedpure Ni. The sliding wear behavior is evaluated with a pin-on-disktribometer. The higher C/Ti atomic ratio of the Ni-3Ti-20C compositeresulted in significantly lower steady-state friction coefficient (0.12)and wear factor (6.8×10⁻⁷ mm³/Nm) than the Ni-10Ti-10C composite andpure Ni. The corresponding Vickers hardness numbers are 165, 240 and 370for pure Ni, Ni-3Ti-20C. and Ni-10Ti-10C composite, respectively. Whilethe Ni-10Ti-10C composite has the highest mechanical hardness due toincreased amount of TiC phase, the Ni-3Ti-20C composite still retainsrespectable hardness due to some TiC phase. Thus, the acceptable rangeC/Ti atomic ratio of self-lubrication phase (“C”) to reinforcement phase(“TiC”) is about 7 to 4.

The novelty of this Ni-3Ti-20C composite is that it exhibitssolid/self-lubricating behavior, forming an in situ, low interfacialshear strength, disordered carbon film during sliding, resulting in adecrease in friction coefficient and wear factor compared to aNi-10Ti-10C composite (high hardness only) and pure Ni (high fracturetoughness only). These chemistry and microstructural phases wereexamined with advanced electron microscopy and chemical spectroscopytechniques. FIG. 5 shows planar surface Auger electron spectroscopychemical map of the worn Ni-3Ti-20C composite. The formation of an insitu, low interfacial shear strength carbon film was confirmed insidethe wear track. FIG. 6 shows the complementary subplanar (subsurface)focused ion beam cross-section inside the worn surface. This scanningelectron microscopy structural image shows that the sheared primarygraphite is fed to the surface during the wear process. Thismicrostructural evolution during wear proves the solid/self-lubricationmechanism of the composite. Zone 1 labeled in FIG. 6 shows that the Nigrains also exhibit strain-induced grain refinement (smaller grains) dueto the tribological shearing process. Zone 2 transitions into theundeformed (non-sheared) Ni, TiC and C phase region.

The LENS deposited titanium carbide/graphite/nickel hybrid composite canbe processed in near-net shape for direct use in a wide array ofengineering operational parts, e.g., oil-drilling components (wear band,stabilizer, and drill collar). LENS has applicability in an industrialsetting. It is also a low cost, low maintenance fabrication process.Relatively large shapes/parts (up to cubic foot) can be fabricated withLENS.

REFERENCES CITED

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

-   (1) C. Atwood, M. Griffith, L. Harwell, E. Schlienger, M. Ensz, J.    Smugeresky, T. Romero, D. Greene, D. Reckaway: Proceedings of the    Laser Material Processing Conference (ICALEO '98), 1998, Orlando,    Fla., USA, pp. E1-E7-   (2) X. Wu, J. Mei, J. Mater Proc Tech, 2003, vol. 135, pp. 266-70.-   (3) T. W. Scharf, A. Neira, J. Y. Hwang, J. Tiley, R. Banerjee: J    Appl Phys, 2009, vol. 106, pp. 013508-7.

What is claimed:
 1. A monolithic hybrid composite comprising: acomposite matrix; a reinforcement phase; and a solid/self-lubricationphase.
 2. The monolithic hybrid composite of claim 1, wherein thecomposite matrix is a nickel matrix.
 3. The monolithic hybrid compositeof claim 2, wherein the nickel matrix is processed using pure nickelpowder having a diameter of between about 40-150 μm.
 4. The monolithichybrid composite of claim 1, wherein the solid/self-lubrication phase isgraphitic carbon.
 5. The monolithic hybrid composite of claim 4, whereinthe graphitic carbon is processed using nickel-coated graphite powderhaving a diameter of between about 40-150 μm.
 6. The monolithic hybridcomposite of claim 1, wherein the reinforcement phase is titaniumcarbide.
 7. The monolithic hybrid composite of claim 6, wherein thetitanium carbide is formed by reacting the pure titanium powder, havinga diameter of between about 40-150 μm, with the graphitic carbon powderduring the process.
 8. The monolithic hybrid composite of claim 1,wherein the ratio of the self-lubrication phase to the reinforcementphase, in atomic ratio, is about 7 to
 4. 9. A method of fabricating amonolithic hybrid composite, comprising the step of: blending elementalnickel powder and titanium powder together with nickel-coated graphitepowder.
 10. The method of claim 9, wherein the atomic percent ratio ofnickel:titanium:nickel-coated graphite powders are approximately77:3:20, or Ni-3Ti-20C.
 11. The monolithic hybrid composite of claim 10exhibits optimum fracture toughness, hardness, andsolid/self-lubricating properties.